A Japanese subcontractor working for the stricken Fukushima nuclear plant has admitted it asked workers to lie about radiation exposure.

Fourteen employees from construction firm Build-Up, a subcontractor to plant owner Tokyo Electric Power Company (TEPCO), were working at the plant between November and March this year after it was crippled by the 2011 earthquake and tsunami.

The company has admitted that one of its executives told workers to cover their radiation devices, called dosimeters, with lead casings in an effort to give false readings of the amount of radiation they were exposed to.

Dosimeters can be worn as badges or carried as devices about the size of a smartphone to detect radiation.

Employees were told that if they did not fake the exposure levels, they would rapidly exceed the legal limit of 50 millisieverts and they would have to stop working.

A TEPCO spokesman said on Saturday that the company was aware from a separate contractor that Build-Up made the lead shields, but that they were never used at the nuclear plant.